The basic techniques for electromagnetic logging for earth formations are well known. For instance, using a logging tool to determine resistivity (or its inverse, conductivity) of earth formations adjacent a borehole has long been a standard and important technique in the search for and recovery of hydrocarbons. Generally, a transmitter transmits an electromagnetic signal that passes through formation materials around the borehole and induces a signal in one or more receivers. The properties of the signal received, such as its amplitude and/or phase, are influenced by the formation resistivity, enabling resistivity measurements to be made. The measured signal characteristics and/or formation properties calculated therefrom may be recorded as a function of the tool's depth or position in the borehole, yielding a formation log that can be used to analyze the formation.
It is often useful to determine the dielectric constant and resistivity of a downhole formation when performing subterranean operations. A High Frequency Dielectric Tool (“HFDT”) may be used to measure the apparent dielectric constant and resistivity of a downhole formation. Typically, the HFDT measurements may be used in conjunction with the Complex Refractive Index Method (“CRIM”) to obtain an estimation of the water-filled formation porosity. Specifically, the CRIM equation defines a relationship between the measured (apparent) complex dielectric constant of the formation, the complex dielectric constant of the mud filtrate, the total porosity and the true formation matrix dielectric constant.
However, an implicit assumption when solving the CRIM equation is that the water within the measurement volume of HFDT consists of water from the drilling mud. This assumption is based on two factors. The first factor is the knowledge that water-based-muds (“WBM”) invade the near wellbore region displacing some of the original fluids. Secondly, the HFDT has a shallow depth of investigation (typically, of the order of a few inches) which is often subject to the WBM invasion. This implicit assumption may lead to an inaccurate analysis of the subterranean formation. Mud invasion processes are often complex and the water in the HFDT measurement volume may in reality be a variable mixture of mud filtrate and formation water. Moreover, in instances where the near well bore water resistivity and dielectric constant are different from that of the mud filtrate, the CRIM calculated porosity using known mud filtrate properties may disagree with the real formation porosity.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.